This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Urea cycle disorders (UCD) represent a group of inborn errors of metabolism that usually present catastrophically with neonatal hyperammonemia and have a high mortality and morbidity. Current knowledge about the disorders is limited to a combination of case reports and small studies in isolated centers. This multicenter study intends to further knowledge in several areas: 1) incidence and prevalence, 2) morbidity and mortality, 3) outcome of liver transplantation compared to treatment with drugs and diet, 4) disease severity over time and 5) environmental influences on hyperammonemic episodes.